A semiconductor laser device is used as a light source for the excitation of a solid-state laser device, such as Nd:YAG (Neodymium: Yttrium Aluminum Garnet) or GGG (Gadolinium Gallium Garnet) laser. This is because a semiconductor laser device is superior in size and reliability to a flash lamp which has been conventionally used.
FIG. 2(a) shows, in cross-section, a prior art broad-area type semiconductor laser device used as a light source for the excitation of a solid-state laser. In FIG. 2(a), denoted at 1 is a p type GaAs substrate. A p type Al.sub.0.5 Ga.sub.0.5 As cladding layer 2 is disposed on the substrate 1. An undoped Al.sub.0.1 Ga.sub.0.9 As active layer 3 is disposed on the p type cladding layer 2. An n type Al.sub.0.5 Ga.sub.0.5 As cladding layer 4 is disposed on the active layer 3. An n type GaAs contact layer 5 is disposed on the n type cladding layer 4. A SiO.sub.2 insulating film 6 is disposed on portions of the contact layer 5. An n side electrode 7 is disposed on the contact layer 5 and the insulating film 6, and a p side electrode 8 is disposed on the rear surface of the substrate 1.
FIG. 2(b) is a diagram showing the light intensity distribution at the resonator facet during the oscillation of the broad-area type semiconductor laser.
In order to excite a solid-state laser, laser light emitted from a semiconductor laser device is required to be effectively irradiated a the solid-state laser crystal. When a side pumping type excitation is to be conducted, since a semiconductor laser device is arranged several millimeters from the solid-state laser rod in the transverse direction, the beam emitted from the semiconductor laser device need not be a single lobe, such as in a semiconductor laser device for use with an optical disc, but may have multiple lobes. Therefore, for exciting a solid-state laser, a broad-area type semiconductor laser device as shown in FIG. 2(a) which has high light output power, although the emitted laser beam does not have a single lobe, is often selected.
This structure is obtained by producing a current confinement mechanism including an insulating film 6 after one crystal growth step on the semiconductor substrate 1. This means that the production process is quite simplified.
This broad-area type semiconductor laser device will operate as follows.
When a positive voltage and a negative voltage are applied to the p side electrode 8 and the n side electrode 7 of the semiconductor laser device, respectively, a current flows in a forward direction with respect to the pn junction and electrons and holes are injected into the active layer 3. Since the energy band gap of the active layer 3 is smaller than those of the cladding layers 2 and 4, carriers are confined in the active layer 3. When the carrier density is increased by increasing the current, an induced emission arises, and when the current exceeds a certain threshold, a laser oscillation is started. Once the laser oscillation is started, laser light is emitted to the outside, where the laser output is linear with the current.
In a broad-area type semiconductor laser device shown in FIG. 2(a) since the current path is limited by the SiO.sub.2 insulating film 6, the laser oscillation arises at the active layer region directly below an aperture portion of the SiO.sub.2 film 6. In a broad-area type semiconductor laser device for exciting a solid-state laser, the aperture width of the SiO.sub.2 film 6 is about 100 to 200 microns.
In the prior art broad-area type semiconductor laser device of such a construction, since there is no mechanism for confining laser light in a horizontal direction with respect to the substrate, local laser oscillation, i.e., a filamentation, is generated due to a slight non uniformities within the active layer. This filamentation causes the following drawbacks.
(1) The laser oscillating region changes along with an increase in current, and the near-field pattern, and in its turn, the far-field pattern changes.
(2) A region where laser oscillation does not occur even in the active layer region directly below the aperture portion of the SiO.sub.2 film is generated. The current injected into this region turns into spontaneously emitted light and is again absorbed by the active layer increasing the laser temperature. This unfavourably enhances thermal saturation of the laser light output.
In general, the maximum light output power of the broad-area type semiconductor laser is determined by the thermal saturation, i.e., the phenomenon that the temperature of the active layer is increased carriers which do not contribute to the laser oscillation are increased, and at last the laser light output is saturated. Therefore, the above-mentioned drawback (2) becomes a great problem.